Harmonic regulator for current source rectification and inversion

ABSTRACT

A methods and systems for generating rectified signals are disclosed. For example, a system performing the methods includes a current source rectifier which has a plurality of switches configured to receive an input current from an AC voltage source and to receive a plurality of control signals. The switches are configured to produce a rectified output current based on the input current and the control signals. The system also includes a rectifier controller configured to receive a current sense signal indicative of the rectified output current and to generate the control signals based at least in part on the current sense signal, where the control signals cause the current source rectifier to attenuate at least one of a plurality of harmonic frequencies in the rectified output current.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/029,479, titled “HARMONIC REGULATOR FOR CURRENT SOURCE RECTIFICATIONAND INVERSION,” filed Jul. 6, 2018, which is a continuation of U.S.application Ser. No. 14/934,036, titled “HARMONIC REGULATOR FOR CURRENTSOURCE RECTIFICATION AND INVERSION,” filed Nov. 5, 2015, which is acontinuation of U.S. application Ser. No. 13/900,321, titled HARMONICREGULATOR FOR CURRENT SOURCE RECTIFICATION AND INVERSION,” filed May 22,2013, which claims the benefit of U.S. Provisional Patent ApplicationNo. 61/650,469, titled “HARMONIC REGULATOR FOR CURRENT SOURCERECTIFICATION AND INVERSION,” filed May 22, 2012, which are herebyincorporated in their entirety and for all purposes.

FIELD OF THE INVENTION

The disclosure herein relates generally to three-phase voltagerectifiers, and more particularly to three-phase voltage rectifiers withlow harmonic load presented to the driving power supply.

BACKGROUND OF THE INVENTION

AC to DC converters are frequently used to provide power from an ACpower supply to one or more devices which require DC power. In someapplications, AC to DC converters are used to provide DC power to a DCto AC converter. In such applications, the AC to DC converter and the DCto AC converter collectively form an AC to AC inverter. FIG. 1illustrates a system 100 including an AC to DC converter 120 providingpower from AC service 115 to an AC motor 180 via the DC to AC converter130.

In some cases, the AC service 115 can have power quality requirements,such as those specified by Mil-Std-1399 Sec. 300B, or other powerquality standards. In such cases, harmonics generated by the AC to DCconverter 120 should be minimized, as they may cause the voltage of theAC service 115 to deviate from a pure sinusoid. These deviations maycause poor performance in other devices connected to the AC service 115.In addition, these deviations may cause the AC service 115 to fail thepower quality requirements.

AC to DC Converters may utilize pulse-width modulated current sourcerectifiers, such as that shown in FIG. 2, as opposed to passiverectifiers, thyristor-based phase controlled rectifiers and voltagesource rectifiers. The current source rectifier is an attractivealternative to these other types of rectifiers more widely used byindustry because it can achieve AC-to-DC voltage conversion with nearlysinusoidal input currents with less input filter components, no in-rushlimiting circuitry and no step-down transformer in cases where thedesired output voltage is lower than the input. All of this results insmaller size and lower parts count.

The current source rectifier 110 shown in FIG. 2 is an active AC-to-DCrectifier that converts three-phase AC input power to a controlled DCvoltage through an active rectification process and drives a constantpower load 104. In the illustration, the three-phase AC input power isshown coming off of an AC power grid 102 and is provided to the currentsource rectifier 110 through input lines a, b, and c. The controller 101receives the input voltages via signal drivers 106 and 108. Thecontroller 101 also receives output voltage v₀ as well as the DC linkcurrent i_(p) through the inductor L_(P) from the current sourcerectifier 110. Controller 101 then provides control signalsU_(i1)-U_(i6) to control the switches S_(i1)-S_(i6) of the currentsource rectifier 110, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a system providing power to amotor.

FIG. 2 is a simplified block diagram of a conventional controller andcurrent source rectifier.

FIG. 3 is a simplified block diagram of a driver having a current sourcerectifier with a rectifier control circuit, according to an embodimentof the present invention.

FIG. 4 is a phasor diagram for use with certain embodiments.

FIG. 5 is a simplified schematic diagram of an example of a currentsource rectifier in a first switch state according to one embodiment.

FIG. 6 is a simplified schematic diagram of an example of a currentsource rectifier in a second switch state according to one embodiment.

FIG. 7 is a simplified schematic diagram of an example of a currentsource rectifier in a third switch state according to one embodiment.

FIG. 8 is a simplified graph of input-to-output voltage ratios of acurrent source rectifier according to one embodiment.

FIG. 9 is a simplified block diagram of a rectifier control circuit fora current source rectifier, according to an embodiment of the presentinvention.

FIG. 10 is a simplified block diagram of a DC offset removal circuit fora harmonic regulator, according to an embodiment of the presentinvention.

FIG. 11 is a simplified block diagram of a harmonic regulator, accordingto an embodiment of the present invention.

FIG. 12 illustrates the current, voltage, and harmonic performance withand without harmonic regulation.

FIG. 13 illustrates input current and DC link current with and withoutharmonic regulation. is graph showing harmonic content of input currentwithout harmonic regulation of the DC Link current according to anembodiment of the invention.

FIG. 14 illustrates harmonic components of input current and DC linkcurrent with and without harmonic regulation.

SUMMARY OF THE INVENTION

One inventive aspect is a system, which includes a current sourcerectifier having a plurality of switches configured to receive an inputcurrent from an alternating current (AC) voltage source and to receive aplurality of control signals. The switches are configured to produce arectified output current based on the input current and the controlsignals. The system also includes a rectifier controller configured toreceive a current sense signal indicative of the rectified outputcurrent and to generate the control signals based at least in part onthe current sense signal, where the control signals cause the currentsource rectifier to attenuate at least one of a plurality of harmonicfrequencies in the rectified output current.

Another inventive aspect is a rectifier control circuit, configured tocontrol a current source rectifier configured to generate a rectifiedoutput current. The rectifier control circuit is further configured toreceive a current sense signal indicative of the rectified outputcurrent and to generate a plurality of control signals based at least inpart on the current sense signal, where the control signals cause thecurrent source rectifier to attenuate at least one of a plurality ofharmonic frequencies in the rectified output current. The rectifiercontrol circuit includes a DC offset circuit configured to remove a DCcomponent from the current sense signal to generate an AC sense signal,a harmonic angle generator configured to generate a plurality ofharmonic frequency signals, and one or more harmonic regulator circuits,each configured to receive the AC sense signal and one of the harmonicfrequency signals, and to generate a harmonic compensation signal basedon the received harmonic frequency signal and the AC sense signal. Therectifier control circuit also includes a combiner configured togenerate a feedback signal based on the harmonic compensation signals ofthe harmonic regulator circuits, and a power switch state selectioncircuit configured to receive the feedback signal and to generate thecontrol signals based on the feedback signal.

Another inventive aspect is a variable speed drive, including a currentsource rectifier having a plurality of switches configured to receive aninput current from an alternating current (AC) voltage source and toreceive a plurality of control signals, where the switches areconfigured to produce a rectified output current based on the inputcurrent and the control signals. The variable speed drive also includesa rectifier controller configured to receive a current sense signalindicative of the rectified output current and to generate the controlsignals based at least in part on the current sense signal, where thecontrol signals cause the current source rectifier to attenuate at leastone of a plurality of harmonic frequencies in the rectified outputcurrent. The variable speed drive also includes an inverter configuredto receive the rectified output voltage and to generate and AC voltageoutput based on the rectified output voltage.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Typically, current source converters do not exhibit many of theundesirable side-effects of power conversion that can be found involtage source converters (used in many variable speed drives and powerconverters) because they do not impose voltage pulses on the source orload. As such, current source converters do not typically require largeinput filter inductors and capacitors that may be necessary for voltagesource converters (VSC) to reduce electro-magnetic interference (EMI) ortransmission line effects caused by high voltage changes with respect totime (dV/dt). Current source converters may also present nearlysinusoidal voltages to the system, so that filter size requirements formeeting low input current harmonic distortion can be less burdensomethan those for voltage source converters.

The Harmonic Regulation System described herein enables theimplementation of a control that targets specific harmonics. By reducingthe magnitude of these harmonics, the magnitude of the harmonic contentpresented to the power distribution system at the input of the convertercan be maintained below system limits. In addition, headroom can beallocated to allow for higher ripple current in the DC link inductor.

The harmonic regulation system discussed below can achieve the sameperformance as a very high sample rate system but with more reasonablecontrol update execution rates for the calculation of U_(i1)-U_(i6) tocontrol the switches S_(i1)-S_(i6) of the current source rectifier formicroprocessor or digital signal processor controlled systems.Therefore, the harmonic regulation system discussed below enables theuse of low cost processing hardware.

Certain embodiments of the invention provide for control of harmoniccurrents at the inputs of a variable speed drive through control of asingle quantity representing DC Link Current. This may require lesssensing hardware than other conventional techniques that monitor thethree-phase input voltages and currents and use the monitored parametersto reduce harmonics or to implement an active damping approach. Theresult can be significantly lower cost and greater simplicity.

In some embodiments, it is also possible to enable harmonic regulatorsto remove voltage harmonics that would otherwise be imposed upon thepower distribution system.

FIG. 3 illustrates a variable speed drive 200, according to anembodiment of the invention. The variable speed drive 200 may, forexample, be used in the system 100. The variable speed drive 200 isconnected to a three-phase voltage source 210, and is configured toprovide power from the voltage source 210 to a motor 270. Variable speeddrive 200 includes power switches 220, a DC link component, inductor230, a rectifier controller 240, a voltage inverter 250, and invertercontroller 260. Embodiments of the invention provide for theimplementation of a power efficient current source rectifier, having aphysically smaller DC link inductor, using low cost, practicallyimplementable control hardware.

The voltage source can be a 60 Hz three-phase alternating current (AC)source, or the like. The power switches 220 may, for example, beinsulated gate bipolar transistors (IGBTs), Metal Oxide Field EffectTransistors (MOSFETS), or the like. Some embodiments include additionalfiltering means to help reduce harmonic currents. For example,additional capacitors, resistors, or combinations thereof, can beadditionally used to filter unwanted frequencies.

Three-phase voltage source 210 provides current to the power switches220 on three input lines. Each of the input lines provides one phase ofcurrent to a pair of power switches 220. The power switches 220, as partof a current source rectifier, provide current to the motor 270 throughDC link inductor 230 and voltage inverter 250.

The voltage inverter 250 receives a substantially DC voltage through thefiltering action of the DC link inductor 230 and the DC link capacitor270. The motor receives a three-phase voltage having a frequency basedon the operation of the voltage inverter 250, which is configured toselectively turn on and off the switches of the voltage inverter 250.The switches of the voltage inverter 250 are turned on and off withsignals from the inverter controller 260 which are determined accordingto a pulse width modulation scheme. The timing and duration of thesignals are selected to provide a substantially sinusoidal three-phasevoltage to motor 270. As shown in FIG. 3, voltage inverter 250 is avoltage source inverter. In other embodiments, a current source invertermay alternatively be used.

The operation of the power switches 220 is controlled by the rectifiercontroller 240. Using a pulse width modulation scheme, the rectifiercontroller 240 provides signals to the power switches 220 to selectivelyturn on and off the power switches 220 according to the phases of thethree input voltages. The timing and duration of the signals areselected to provide a rectified current to DC link inductor 230 and arectified DC voltage to the voltage inverter 250.

In this embodiment, the rectifier controller 240 receives a signal (Vb)representing the voltage at the voltage inverter 250. As discussedfurther below, the rectifier controller is configured to control thetiming and duration of the signals controlling the power switches based,at least in part, on the signal representing the signal Vb. For example,the rectifier controller 240 is at least configured to control thetiming induration signals such that Vb is substantially fixed and isapproximately equal to a reference voltage.

The rectifier controller 240 also receives a signal (Ib) representingthe current of the DC link inductor 230. Other signals may be used inother embodiments such as the AC voltage feedbacks Vabc which are fedinto a harmonic angle generator in order to lock the timing of theselection of gate signals to the fundamental frequency of the periodicchange in the input voltages.

As discussed further below, the rectifier controller 240 is configuredto control the timing and duration of the signals controlling the powerswitches based, at least in part, on the AC voltage signals and thesignal Ib. For example, the rectifier controller 240 is at leastconfigured to control the timing and duration of the signals such thatcertain harmonics in the current of the DC link inductor 230 areattenuated.

FIGS. 4-8 illustrate examples of aspects of the functionality of therectifier controller 240 and the power switches 220, which operate withthe DC link inductor 230 and the DC link capacitor 270 to generate asubstantially DC voltage. The functionality of the rectifier controller240 may alternatively operate according to different aspects andprinciples. FIGS. 9-11 illustrate examples of aspects of the rectifiercontroller 240 and the power switches 220, which work with DC linkinductor 230 and the DC link capacitor 270 to attenuate harmonics in thecurrent of the DC link inductor 230. The functionality of the rectifiercontroller 240 may alternatively operate according to different aspectsand principles.

FIG. 4 illustrates a phasor diagram which divides the electrical cycleof the periodically changing incoming three-phase voltages and currentsθ_(i) into six equal 60 degree sectors. As can be seen, the currentsource rectifier switch states have been superimposed on the abc phaseplane of the phasor diagram. Three (or four) active switches of thecurrent source rectifier and the two applied line-to-line voltages orthree applied line-to-neutral voltages change every 60 degree sector.S_(k) and S_(n) denote the two PWM-controlled switches, S_(q) denotesthe switch that is held on, and S_(z) denotes the switch for the zerostate (if used). One embodiment of the current source rectifier control,synthesizes sinusoidal three-phase input currents and controls DC outputvoltage by controlling the amount of time PWM-controlled switches S_(k)and S_(n) are turned on over a switching frequency period according toequations 1-3 below, where the duty cycles are given by equations 4-6.

$\begin{matrix}{T_{k} = {{d_{k}\left( \phi_{i} \right)} \cdot T_{si}}} & 1 \\{T_{n} = {{d_{n}\left( \phi_{i} \right)} \cdot T_{si}}} & 2 \\{{T_{o} = {\left\lbrack {1 - {d_{k}\left( \phi_{i} \right)} - {d_{n}\left( \phi_{i} \right)}} \right\rbrack \cdot T_{si}}}{where}} & 3 \\{{d_{k}\left( \phi_{i} \right)} = {U_{eq} \cdot {\sin \left( {\frac{\pi}{3} - \phi_{i}} \right)}}} & 4 \\{{d_{n}\left( \phi_{i} \right)} = {U_{eq} \cdot {\sin \left( \phi_{i} \right)}}} & 5 \\{T_{si} = \frac{1}{f_{si}}} & 6\end{matrix}$

The angle φ_(i) is related to θ_(i) by the following relationship:

$\begin{matrix}{{\phi_{i}(t)} = {\theta_{i} + \frac{\pi}{6}}} & 7\end{matrix}$

Where θ_(i) is produced by the harmonic angle generator and φ_(i) resetsto zero at

$\begin{matrix}{{\theta_{i} = \frac{\pi}{6}},\frac{\pi}{2},\frac{5 \cdot \pi}{6},\frac{7 \cdot \pi}{6},\frac{3 \cdot \pi}{2},\frac{11 \cdot \pi}{6}} & 8\end{matrix}$

and each of the above values of θ_(i) represents entry into a subsequent60 degree sector and exit from a previous 60 degree sector.

One way to generate the gate control signals U_(i1)-U_(i6) is to comparethe duty cycles of equations 4 and 5 to symmetrical triangle waves thatvary with the switching frequency f_(si) and are 180 degrees out ofphase with each other in order to produce the gating signals for thePWM-controlled switches S_(k) and S_(n). Table 1 below shows how theswitches of FIG. 3 are assigned in each sector. Those switch states thatare generated through comparison with 180 degree phase-shifted trianglesare designated by the shaded cells with * marks under the S_(k) andS_(n) headings for each sector.

TABLE 1 Pole voltages and currents in each sector PWM Electrical SectorControlled Angle Switches Switches Pole Voltages Pole Currents (□_(i))Sector S_(q) S_(z) S_(k) S_(n) v_(k) v_(n) v_(k0) v_(n0) i_(k) i_(n) 330° 7 S_(i1) S_(i4) * S_(i6)  S_(i2)  v_(ab) −v_(ca) −v_(b)  −v_(c) −i_(b)  −i_(c)   30° 2 S_(i2) S_(i5)  S_(i1) * S_(i3) −v_(ca)  v_(bc)v_(a) v_(b) i_(a) i_(b)  90° 3 S_(i3) S_(i6) * S_(i2)  S_(i4)  v_(bc)−v_(ab) −v_(c)  −v_(a)  −i_(c)  −i_(a)  150° 4 S_(i4) S_(i1)  S_(i3) *S_(i5) −v_(ab)  v_(ca) v_(b) v_(c) i_(b) i_(c) 210° 5 S_(i5) S_(i2) *S_(i4)  S_(i6)  v_(ca) −v_(bc) −v_(a)  −vb  −i_(a)  −i_(b)  270 6 S_(i6)S_(i3)  S_(i5) * S_(i1) −v_(bc)  v_(ab) v_(c) v_(a) i_(c) i_(a)

As discussed above, a current source with a freewheeling diode does notrequire the switching of S_(z) in order to create the zero state.Instead, the freewheeling diode automatically creates the zero state byproviding a path through which DC link inductor current can flow whenthe devices are switched off.

PWM operation of the current source rectifier can be understood byrestricting the analysis to a single 60° sector. FIGS. 5-7 show thecurrent source rectifier circuit 110 of FIG. 3 during its threecommutation stages for a single sector (Sector 2) with its two activeswitch states (FIGS. 5 and 6) and the zero or null state (FIG. 7). Thezero state can also be produced with a freewheeling diode in which casethe two active switches are in the off state. The two active switchstates are named for the active switch, denoted as either the switch orthe ‘n’ switch. The sector is named for the switch that is held on, theq′ switch. The zero state is produced by switching on the ‘z’ switch.This ‘k-n-q-z’ convention is followed from this point forward in thediscussion herein. The ‘k-n-q-z’ switches change every sector, as do theinput pole voltages (the line-to-line voltage across the associatedinput capacitor) designated as v_(k) and v₀ and the pole currents (thecurrents that flow through the active switch when it is on) designatedas v_(k) and v₀. Table 1 above shows the pole voltages and currents thatare assigned to v_(k), v_(n), i_(k) and i_(n) for each sector.

FIGS. 5-7 depict example block diagrams of a current source rectifieroperating in different switch states according to one embodiment. Theassignment of the applied line-to-line voltages v_(k) and v_(n), inputcurrents i_(k) and i_(n), and equivalent line-to-neutral voltages v_(k0)and v_(n0) for each sector is shown in Table 1 above. In a given sector,the current source rectifier PWM-controlled switches (S_(k) and S_(n))and full on switch (S_(q)) connect two AC input voltages to the DC linkinductor L_(p), and synthesize a DC voltage through PWM control that islower than the peak line-to-line voltage of the two voltages.

FIG. 5 depicts an example of a current source rectifier in a firstswitch state according to one embodiment. In the illustrated embodiment,this active switch state is referred to as the ‘k’ switch state. In thisstate, the input voltage across the DC link inductor L_(p) is v_(k) andthe current i_(k)=i_(p) flows through L_(p). In order to reach thisstate, switches S_(k) (e.g., S_(i1) from FIG. 3) and S_(q) are turned onwhile the other switches in the current source rectifier remain turnedoff. FIG. 6 depicts an example of a current source rectifier in a secondswitch state according to one embodiment. In the illustrated embodiment,this active switch state is referred to as the ‘n’ switch state. In thisstate, the input voltage across the DC link inductor L_(p) is v_(n) andthe current i_(n)=i_(p) flows through L_(p). In order to reach thisstate, switches S_(n) (e.g., S_(i3) from FIG. 3) and S_(q) are turned onwhile the active switches in the current source rectifier remain turnedoff. FIG. 7 depicts an example of a current source rectifier in a thirdswitch state according to one embodiment. In the illustrated embodiment,this switch state is referred to as the ‘z’ or ‘zero’ switch state. Inthis state, there is no input voltage applied across the DC linkinductor L_(p) and thus current i_(p) flows to ground and dissipates inthe load. In order to reach this state, switches S_(z) (e.g., S_(i5)from FIG. 3) and S_(q) are turned on while the other active switches inthe current source rectifier remain turned off.

FIG. 9 illustrates a rectifier control circuit 300 according to anembodiment of the invention. The rectifier control circuit 300 can beused, for example, in the rectifier control circuit 240 of variablespeed drive 200 of FIG. 2 to generate the signals discussed above withreference to FIGS. 4-8 such that certain harmonics in the current of theDC link inductor 230 are attenuated.

The rectifier control circuit 300 includes a phase locked loop (PLL) 370as a harmonic angle generator, a DC offset removal block 360, a 2^(nd)harmonic regulator 350, a 3^(rd) harmonic regulator 340, a 6^(th)harmonic regulator 330, a power switch state selection circuit 320, anda proportional plus integral regulator (PI regulator) 310. The specificimplementation for each of the phase locked loop (PLL) 370, the DCoffset removal block 360, the harmonic regulators 330, 340, and 350, thepower switch state selection circuit 320, and the PI regulator 310 isnot limited. Some embodiments include more or fewer harmonic regulatorsand some embodiments regulate other harmonics (e.g., 1^(st), 4^(th), and8^(th) harmonics).

In operation, the DC link current signal (Ib) is provided to the DCoffset removal block configured to remove a DC component from thecurrent sense signal to generate an AC sense signal representing theunwanted harmonic content in the DC link current. Accordingly, theoutput of the DC offset removal block 360 is the AC component of Ib,which is provided to each of the harmonic regulators 330, 340, and 350.

The harmonic regulators 330, 340, and 350 respectively generate aharmonic compensation signal based on the frequency component of the Ibsignal matching the harmonic which each of the regulators 330, 340, and350 are configured to respond to. The respective harmonic compensationsignals are combined with a unity signal at summing circuit 380 togenerate a harmonic compensation signal, which is used as discussedbelow.

PI regulator 310 receives a DC error voltage based on a differencebetween reference voltage Vb* and signal Vb representing the voltage atvoltage inverter 250. Based on the error voltage, PI regulator 310generates a difference signal Ud.

Multiplier 390 combines the difference signal Ud with the harmoniccompensation signal to generate a feedback control signal. Because thedifference signal Ud is based on the DC error voltage, and the harmoniccompensation signal is based on the harmonics in the DC link currentsignal Ib to be attenuated, the feedback control signal is based on boththe DC error voltage and the harmonics to be attenuated. The feedbackcontrol signal is provided to power switch state selection circuit 320.

The power switch state selection circuit 320 generates power switchstate signals based on the feedback control signal to generate thesignals for the power switches 220. Accordingly, the power switch stateselection circuit 320 generates power switch state signals which resultin a substantially DC voltage as discussed above with reference to FIGS.4-8, and which result in attenuation of harmonics as discussed abovewith reference to FIG. 9.

As a result, the states of the power switches are controlled such that asubstantially fixed DC voltage which is substantially equal to thereference voltage is provided across bypass capacitor 270, and such thatthe current through DC link inductor 230 has low harmonic distortion.

FIG. 10 illustrates a DC offset removal circuit 400 according to anembodiment of the invention. The DC offset removal circuit 400 isconfigured to generate and output signal which has substantially onlythe AC components of the input signal. The DC offset removal circuit 400can be used, for example, in the rectifier control circuit 300 of FIG. 3to remove the DC component of the DC link current signal Ib. Othercircuits may alternatively be used for the DC offset removal circuit 360of FIG. 3.

The DC offset removal circuit 400 includes a forward difference filter410 and a summer 420. The input signal Ib is provided to a positiveinput of summer 420. In addition the input signal Ib is provided to theforward difference filter 410. The forward difference filter 410generates an output voltage Ibdc, which is provided to a negative inputof summer 420. The summer 420 combines the Ib and Ibdc signals andgenerates an output Ibh. Ibh contains the AC components of the inputsignal Ib.

FIG. 11 illustrates a harmonic regulator circuit 500 according to anembodiment of the invention. The harmonic regulator circuit 500 isconfigured to The harmonic regulator circuit 500 can be used, forexample, in the rectifier control circuit 300 of FIG. 3. Other circuitsmay alternatively be used for the rectifier control circuit 300 of FIG.3.

Harmonic regulator circuit 500 receives the AC sense signal representingthe unwanted harmonic content in the DC link current, and a frequencysignal from the PLL 370. Harmonic regulator circuit 500 is configured toextract the amplitudes of two quadrature DC link current harmonics ofthe input fundamental frequency, and to generate two harmoniccompensation signals with two PI regulators, which are summed. Thefeedback reduces the harmonic component of the AC sense signal.

FIGS. 12-15 illustrate the impact of the harmonic regulation on theharmonic content of the DC link current and on the harmonic content ofthe input current, according to some embodiments.

FIG. 12 illustrates the amplitudes of the second, third, and sixthharmonics components in the DC link current before and after harmonicregulation is turned on. In this embodiment, when the current sourcerectifier starts up, the harmonic regulation is not active.Consequently, the amplitudes of the second, third, and sixth harmonicsin the DC link current are significant. After the harmonic regulation isengaged and the system has settled, the amplitudes of the second, third,and sixth harmonics are attenuated. As shown, the harmonic content ofthe DC link current is better regulated and is significantly attenuatedafter the harmonic regulator for is engaged.

As understood by those of skill in the art, the reduction in harmoniccontent of the DC link current reduces the harmonic content of the inputcurrent. Specifically, reduction of the second harmonic in the DC linkcurrent results in a reduction of the third harmonic of the inputcurrent. In addition, reduction of the third harmonic in the DC linkcurrent results in a reduction of the second and fourth harmonics in theinput current. Furthermore, reduction of the sixth harmonic in the DClink current results in a reduction of the fifth and seventh harmonicsin the input current.

FIG. 12 also illustrates the values of the DC link voltage, the inputcurrent, and the DC link current before and after the harmonicregulation is turned on, according to one embodiment.

FIG. 13 illustrates the input current and the DC link current (Ipn)without harmonic regulation on the left and with harmonic regulation onthe right, according to one embodiment. As shown on the left, the DClink current has significant harmonic content. The input current isvisibly distorted from sinusoidal. In addition, the DC link current hasamplitude peaks which are significantly different from one another. Asshown on the right, the input current is clearly more sinusoidal. Inaddition, the peaks of the DC link current do not vary as much as thepeaks of the DC link current shown on the left.

FIG. 14 illustrates harmonic components of the input current and the DClink current (Ipn) without harmonic regulation on the left and withharmonic regulation on the right, according to one embodiment.

As shown, the harmonic content of the input current has beensignificantly reduced by the harmonic regulation. In this example, the2^(nd) harmonic has been reduced from −25 dB to −37 dB, the 3^(rd)harmonic has been reduced from −60 dB to −63 dB, the 4^(th) harmonic hasbeen reduced from −25 dB to −31 dB, the 5^(th) harmonic has been reducedfrom −32 dB to −48 dB, and the 7^(th) harmonic has been reduced from −31dB to −48 dB.

Throughout the foregoing description, for the purposes of explanation,numerous specific features were set forth in order to provide anunderstanding of the invention. It will be apparent, however, to personsskilled in the art that the discussed and other embodiments may bepracticed without some of presented features. Likewise, it will beapparent to persons skilled in the art that the discussed and otherembodiments may be practiced with other features not discussed.

What is claimed is:
 1. A method of rectifying an input signal, themethod comprising: with a current source rectifier comprising aplurality of switches, receiving an alternating current (AC) inputcurrent and an AC input voltage from an AC voltage source; with thecurrent source rectifier, receiving a plurality of control signals,wherein the control signals cause the switches to generate a rectifiedoutput current based on the input current and the control signals, andwherein the control signals additionally cause the switches to attenuateone or more harmonic frequencies of the AC input voltage in therectified output current; with a rectifier controller comprising aharmonic angle generator circuit and a power switch state selectioncircuit, receiving an input frequency signal indicative of the AC inputvoltage; with the harmonic angle generator circuit, generating one ormore harmonic frequency signals based on the input frequency signal,wherein each of the harmonic frequency signals has a frequencycorresponding with one of the harmonic frequencies of the AC inputvoltage attenuated in the rectified output current; and with the powerswitch state selection circuit, generating the control signals based inpart on the harmonic frequency signals.
 2. The method of claim 1,wherein the AC input voltage is a three-phase AC voltage.
 3. The methodof claim 1, wherein the plurality of switches are metal-oxide fieldeffect transistors (MOSFETs).
 4. The method of claim 1, furthercomprising: with a DC offset circuit, removing a DC component from acurrent sense signal to generate an AC sense signal, wherein the currentsense signal is indicative of the rectified output current of thecurrent source rectifier.
 5. The method of claim 4, further comprising:with one or more harmonic regulator circuits, receiving the AC sensesignal and one of the harmonic frequency signals; and with the one ormore harmonic regulator circuits, generating a harmonic compensationsignal based on the received harmonic frequency signal and the AC sensesignal, wherein the one or more harmonic regulator circuits includes oneor more of: a first harmonic regulator circuit configured to receive theAC sense signal and a first harmonic frequency signal, and to generate afirst harmonic compensation signal based on the first harmonic frequencysignal and the AC sense signal; a second harmonic regulator circuitconfigured to receive the AC sense signal and a second harmonicfrequency signal, and to generate a second harmonic compensation signalbased on the second harmonic frequency signal and the AC sense signal; athird harmonic regulator circuit configured to receive the AC sensesignal and a third harmonic frequency signal, and to generate a thirdharmonic compensation signal based on the third harmonic frequencysignal and the AC sense signal; a fourth harmonic regulator circuitconfigured to receive the AC sense signal and a fourth harmonicfrequency signal, and to generate a fourth harmonic compensation signalbased on the fourth harmonic frequency signal and the AC sense signal; afifth harmonic regulator circuit configured to receive the AC sensesignal and a fifth harmonic frequency signal, and to generate a fifthharmonic compensation signal based on the fifth harmonic frequencysignal and the AC sense signal; a sixth harmonic regulator circuitconfigured to receive the AC sense signal and a sixth harmonic frequencysignal, and to generate a sixth harmonic compensation signal based onthe sixth harmonic frequency signal and the AC sense signal; a seventhharmonic regulator circuit configured to receive the AC sense signal anda seventh harmonic frequency signal, and to generate a seventh harmoniccompensation signal based on the seventh harmonic frequency signal andthe AC sense signal; and an eighth harmonic regulator circuit configuredto receive the AC sense signal and an eighth harmonic frequency signal,and to generate an eighth harmonic compensation signal based on theeighth harmonic frequency signal and the AC sense signal.
 6. The methodof claim 1, further comprising: with the current source rectifier,generating a controlled output voltage; with the rectifier controller,receiving configured to receive a reference voltage; and with therectifier controller generating the control signals based at least inpart on the reference voltage, wherein the control signals cause thecurrent source rectifier to generate the rectified output current suchthat the controlled output voltage is substantially equal to thereference voltage.
 7. The method of claim 6, further comprising: with avoltage regulator circuit of the rectifier controller, generating adifference signal based on a difference between the reference voltageand the controlled output voltage; and with the power switch stateselection circuit, generating the control signals based on thedifference signal.
 8. The method of claim 7, further comprising: with aDC offset circuit of the rectifier controller further, removing a DCcomponent from a current sense signal to generate an AC sense signal,wherein the current sense signal is indicative of the rectified outputcurrent of the current source rectifier, wherein the one or moreharmonic regulator circuits includes one or more of: a first harmonicregulator circuit configured to receive the AC sense signal and a firstharmonic frequency signal, and to generate a first harmonic compensationsignal based on the first harmonic frequency signal and the AC sensesignal; a second harmonic regulator circuit configured to receive the ACsense signal and a second harmonic frequency signal, and to generate asecond harmonic compensation signal based on the second harmonicfrequency signal and the AC sense signal; a third harmonic regulatorcircuit configured to receive the AC sense signal and a third harmonicfrequency signal, and to generate a third harmonic compensation signalbased on the third harmonic frequency signal and the AC sense signal; afourth harmonic regulator circuit configured to receive the AC sensesignal and a fourth harmonic frequency signal, and to generate a fourthharmonic compensation signal based on the fourth harmonic frequencysignal and the AC sense signal; a fifth harmonic regulator circuitconfigured to receive the AC sense signal and a fifth harmonic frequencysignal, and to generate a fifth harmonic compensation signal based onthe fifth harmonic frequency signal and the AC sense signal; a sixthharmonic regulator circuit configured to receive the AC sense signal anda sixth harmonic frequency signal, and to generate a sixth harmoniccompensation signal based on the sixth harmonic frequency signal and theAC sense signal; a seventh harmonic regulator circuit configured toreceive the AC sense signal and a seventh harmonic frequency signal, andto generate a seventh harmonic compensation signal based on the seventhharmonic frequency signal and the AC sense signal; and an eighthharmonic regulator circuit configured to receive the AC sense signal andan eighth harmonic frequency signal, and to generate an eighth harmoniccompensation signal based on the eighth harmonic frequency signal andthe AC sense signal.
 9. The method of claim 6, further comprising, withan inverter receiving the controlled output voltage and generating an ACvoltage output based on the controlled output voltage.
 10. The method ofclaim 9, wherein the inverter is a current source inverter.
 11. Themethod of claim 9, wherein the inverter is a voltage source inverter.12. A method of operating a variable speed drive, the method comprising:with a current source rectifier comprising a plurality of switches,receiving an alternating current (AC) input current and an AC inputvoltage from an AC voltage source; with the current source rectifier,receiving a plurality of control signals, wherein the control signalscause the switches to generate a rectified output current based on theinput current and the control signals, and wherein the control signalsadditionally cause the switches to attenuate a harmonic frequency of theinput current in the rectified output current; with a rectifiercontroller, receiving an input frequency signal indicative of afrequency of the input current; with a harmonic angle generator circuitof the rectifier controller, generating a harmonic frequency signalhaving a frequency corresponding with the harmonic frequency of theinput current attenuated in the rectified output current, and with apower switch state selection circuit of the rectifier controller,generating the control signals based in part on the harmonic frequencysignal; and with an inverter, receiving the rectified output current andto generate an AC voltage output based on the rectified output current.13. The method of claim 12, wherein the AC input voltage is athree-phase AC voltage.
 14. The method of claim 12, wherein theplurality of switches are metal-oxide field effect transistors(MOSFETs).
 15. The method of claim 12, further comprising: with a DCoffset circuit, removing a DC component from a current sense signal togenerate an AC sense signal, wherein the current sense signal isreceived by the rectifier controller and is indicative of the rectifiedoutput current of the current source rectifier; and at least one of:with a first harmonic regulator circuit receiving the AC sense signaland a first harmonic frequency signal, and generating a first harmoniccompensation signal based on the first harmonic frequency signal and theAC sense signal; with a second harmonic regulator circuit, receiving theAC sense signal and a second harmonic frequency signal, and generating asecond harmonic compensation signal based on the second harmonicfrequency signal and the AC sense signal; with a third harmonicregulator circuit, receiving the AC sense signal and a third harmonicfrequency signal, and generating a third harmonic compensation signalbased on the third harmonic frequency signal and the AC sense signal;with a fourth harmonic regulator circuit, receiving the AC sense signaland a fourth harmonic frequency signal, and generating a fourth harmoniccompensation signal based on the fourth harmonic frequency signal andthe AC sense signal; with a fifth harmonic regulator circuit, receivingthe AC sense signal and a fifth harmonic frequency signal, andgenerating a fifth harmonic compensation signal based on the fifthharmonic frequency signal and the AC sense signal; with a sixth harmonicregulator circuit, receiving the AC sense signal and a sixth harmonicfrequency signal, and generating a sixth harmonic compensation signalbased on the sixth harmonic frequency signal and the AC sense signal;with a seventh harmonic regulator circuit, receiving the AC sense signaland a seventh harmonic frequency signal, and generating a seventhharmonic compensation signal based on the seventh harmonic frequencysignal and the AC sense signal; and with an eighth harmonic regulatorcircuit configured to receive the AC sense signal and an eighth harmonicfrequency signal, and generating an eighth harmonic compensation signalbased on the eighth harmonic frequency signal and the AC sense signal.16. The method of claim 12, further comprising: with the current sourcerectifier, generating a controlled output voltage; with the rectifiercontroller, receiving a reference voltage; and with the rectifiercontroller, generating the control signals based at least in part on thereference voltage, wherein the control signals cause the current sourcerectifier to generate the controlled output voltage such that thecontrolled output voltage is substantially equal to the referencevoltage.
 17. The method of claim 16, further comprising: with a voltageregulator circuit of the rectifier controller, generating a differencesignal based on a difference between the reference voltage and thecontrolled output voltage; and with the power switch state selectioncircuit, generating the control signals based on the difference signal.18. The method of claim 17, with a DC offset circuit of the rectifiercontroller, removing a DC component from a current sense signal togenerate an AC sense signal, wherein the current sense signal isindicative of the rectified output current of the current sourcerectifier; and at least one of: with a first harmonic regulator circuitreceiving the AC sense signal and a first harmonic frequency signal, andgenerating a first harmonic compensation signal based on the firstharmonic frequency signal and the AC sense signal; with a secondharmonic regulator circuit, receiving the AC sense signal and a secondharmonic frequency signal, and generating a second harmonic compensationsignal based on the second harmonic frequency signal and the AC sensesignal; with a third harmonic regulator circuit, receiving the AC sensesignal and a third harmonic frequency signal, and generating a thirdharmonic compensation signal based on the third harmonic frequencysignal and the AC sense signal; with a fourth harmonic regulatorcircuit, receiving the AC sense signal and a fourth harmonic frequencysignal, and generating a fourth harmonic compensation signal based onthe fourth harmonic frequency signal and the AC sense signal; with afifth harmonic regulator circuit, receiving the AC sense signal and afifth harmonic frequency signal, and generating a fifth harmoniccompensation signal based on the fifth harmonic frequency signal and theAC sense signal; with a sixth harmonic regulator circuit, receiving theAC sense signal and a sixth harmonic frequency signal, and generating asixth harmonic compensation signal based on the sixth harmonic frequencysignal and the AC sense signal; with a seventh harmonic regulatorcircuit, receiving the AC sense signal and a seventh harmonic frequencysignal, and generating a seventh harmonic compensation signal based onthe seventh harmonic frequency signal and the AC sense signal; and withan eighth harmonic regulator circuit configured to receive the AC sensesignal and an eighth harmonic frequency signal, and generating an eighthharmonic compensation signal based on the eighth harmonic frequencysignal and the AC sense signal.
 19. The method of claim 18, wherein theinverter is a current source inverter.
 20. The method of claim 18,wherein the inverter is a voltage source inverter.